The present invention relates to a DC differential amplifier and, more particularly, to a highly stable DC amplifier having less temperature dependency and a large margin with respect to changes in gain due to absolute value variations of circuit elements such as transistors and resistors constituting the amplifier.
FIG. 3 shows a typical conventional DC differential amplifier. As shown in FIG. 3, in this amplifier, a pair of transistors Q.sub.1 and Q.sub.2 each having a base as an input terminal constitute a differential amplification transistor pair, and their emitters are connected to each other through an emitter resistor R.sub.E and are connected to a constant voltage source V.sub.E through resistors R.sub.1 and R.sub.2. The collectors of the pair of transistors Q.sub.1 and Q.sub.2 are respectively cascode-connected to the emitters of common base transistors Q.sub.3 and Q.sub.4, and are further connected to to a constant voltage source V.sub.C through load resistors R.sub.L1 and R.sub.L2. In this case, the resistors R.sub.1 and R.sub.2 and the constant voltage V.sub.E have predetermined values so as to cause the transistors Q.sub.1 and Q.sub.2 to be driven by a current source. The transistors Q.sub.3 and Q.sub.4 are cascode-connected to each other so as to reduce the Miller effect. However, A DC operation is kept unchanged without them. Note that reference symbol V.sub.B denotes a constant voltage source for applying a constant bias potential to the bases of the transistors Q.sub.3 and Q.sub.4.
In the amplifier having the above-described arrangement, if a load resistor is represented by R.sub.L =R.sub.L1 =R.sub.L2, input and output voltages are respectively represented by V.sub.in and V.sub.out, and the emitter resistance of the transistor is represented by r.sub.E, an output voltage from the half of the differential amplifier can be given by the following equation: EQU V.sub.out ={R.sub.L /(r.sub.E +R.sub.E /2)}.V.sub.in ( 1)
In this case, if the operation state of the transistor is carefully checked, it is found that an amount of heat generated by the transistor (P.apprxeq.I.sub.C .times.V.sub.CE ; I.sub.C : a collector current; V.sub.CE a voltage between the collector and the emitter) is changed upon application of the input voltage V.sub.in. Upon this change, a slight change (.DELTA.V.sub.BE) of a voltage between the base and emitter of the transistor occurs. This change is equivalent to a further change in input voltage V.sub.in. The output voltage V.sub.out at this time can be represented by the following equation: EQU V.sub.out ={R.sub.L /(r.sub.E +R.sub.E /2)}.multidot.(V.sub.in +.DELTA.V.sub.BE) (2)
As described above, the conventional amplifier cannot be free from the influence of the change .DELTA.V.sub.BE. Note that the change .DELTA.V.sub.BE is generally given as EQU .DELTA.V.sub.BE .perspectiveto.-2 mV/.degree.C. (3)